1. Field of the Invention
The present invention relates to wire bonding apparatus for semiconductor devices and more particularly to a wire bonding apparatus fit for use in manufacturing high-density semiconductor devices with accuracy.
2. Prior Art
Wire bonding apparatus and the like are increasingly desired to be precise in dealing with circuit patterns which tend to become miniaturized and densified and also with high-density packaging. It is now deemed essential for such wire bonding apparatus to correspond with the miniaturization of not only bonding pads but also bonding leads on semiconductor chips, for instance.
When leadframes, particularly thin leadframes of multi-pin LSI as intended workpieces, are handled, the well-known practice is to recognize patterns by means of an ITV camera for detecting what has been magnified through an optical lens barrel.
The wire bonding process stated above will be described with reference to FIG. 4.
While mounting a semiconductor chip 2 on the surface, a leadframe 1 as the workpiece having been taken out of a supply magazine (not shown), is conveyed up to a locking position for bonding with both its lateral end-undersurfaces supported by a pair of guide rails 3a, 3b.
A switching pulse motor 4, a flighted screw 5 with left and right portions threaded in opposite directions and the like are utilized for leveling and switching the pair of guide rails 3a, 3b, the guide rails 3a, 3b being contracted at the locking position to laterally position the leadframe by abutting against it.
A block 7 for supporting a workpiece such as a heater block which is made vertically freely moveable by a stroke bearing 6 is arranged under the locking position, whereas a presser member 9 for holding down the leadframe is arranged above the locking position, the presser member 9 being also made vertically freely moveable by a stroke bearing 8. The leadframe 1 is vertically sandwiched between the block 7 for supporting the workpiece and the presser member 9 and fixed at the locking position.
On the other hand, an optical lens barrel 11 for magnifying and guiding an image to a monitoring (CCD) camera 10 is disposed above the locking position of the leadframe 1. The monitoring camera 10 and the optical lens barrel 11 are secured to a bonding head 13 mounted on an X-Y table 12. A bonding arm 15, fitted with a bonding tool 14 at its leading end, is fitted to the bonding head 13 in such a way that it is capable of rocking.
The pattern enlarged through the optical lens barrel 11 is detected by the monitoring camera 10. Then the pattern on a combination of a bonding pad and an inner lead 1a of the leadframe 1 on the semiconductor chip 2 is recognized and the shifting of the bonding position is corrected, whereby wires 16 may be bonded by means of a bonding tool 14 accurately.
In a wire bonding apparatus of the sort stated above, however, the shifting of the optical focal point due to the difference in level between the surface of the semiconductor chip 2 and that of the inner lead 1a of the leadframe 1 may deteriorate the accuracy. As a result, it has generally been arranged that the optical lens barrel 11 is made vertically movable via a rolling bearing to prevent the focal point from shifting.
Moreover, there has been proposed a wire bonding apparatus for hybrid ICs in Japanese Patent Laid-Open No. 125639/1987. FIG. 5 shows the wire bonding apparatus disclosed therein.
A substrate fixing part 7' for holding a substrate 1 as an intended workpiece is made vertically movable via a motor 17 and a flighted screw 18 extended perpendicularly and rotated as the motor 17 rotates, so that the substrate 1 is vertically moved in conformity with the focal point of a monitoring camera 10.
In the case of the conventional wire bonding apparatus shown in FIG. 4, however, the problem is that the shifting of the optical focal point due to the difference in level between the surface of the semiconductor chip and that of the inner lead results in reduced accuracy. If the leadframe is to be vertically moved to solve the aforementioned problem, the block for supporting the workpiece together with the leadframe presser will have to be vertically moved. Even if the block for supporting the workpiece together with the leadframe presser is made vertically movable, the guide rails may rub against the edge faces of the leadframe, thus subjecting the inner lead of the leadframe and the bonded wire to oscillation, which may damage both of them and hence develop structural defects.
With the optical lens barrel arranged vertically movable, moreover, the provision of a mechanism for vertically moving the lens barrel will not only induce an increase in weight and cost but also create factors in reducing accuracy attributable to clearance and backlash in the slide portion of the optical lens barrel.
More specifically, owing to the general mechanical accuracy required to render the mechanism vertically movable, the problem of positional reproducibility of the vertical movement and that of the horizontal positional reproducibility derived from its vertical movement may sacrifice the accuracy. If it is attempted to vertically move the optical lens barrel by approximately 0.5 mm which is equivalent to the thickness of the semiconductor chip, for instance, the optical lens barrel may slightly rock as it vertically moves. Since the horizontal change resulting from the rocking motion is infinitesimal, it becomes hardly correctable.
In other words, a gap of several microns with the rolling bearing, for instance, constitutes an important factor in making inaccurate the bonding position at the stage where the accuracy of the bonding position of the order of microns is narrowed down.
In the case of the conventional wire bonding apparatus shown in FIG. 5, the substrate as the intended workpiece is vertically moved in conformity with the focal point of the optical system in order to adjust the substrate to the reference level of the bonding tool as bonding level after the pattern has been recognized. For this reason, the bonding thus effected is considered inclusive of errors ascribed to the vertical movement of the substrate.
Wire bonding by means of leadframes is not at a problematical level as long as the amount by which the perpendicularity of a bonding tool is affected because the difference in level resulting from the difference in the thickness of the leadframe, the level of a semiconductor chip and the like is within the range of approximately tan .THETA.=0.7 mm/1.10 mm, .THETA.=0.4.degree.. Consequently, if the perpendicularity of the bonding tool is within the above range of errors, the merit of taking the perpendicularity into consideration may be low; on the contrary, the demerit is considered rather high.
Since the flighted screw is employed for driving the substrate to move vertically, it is necessary to vertically move the leadframe presser for fastening the leadframe to the block for supporting the workpiece in addition to the member for fixing the substrate when, for instance, the leadframe is handled. As a result, two drives have to be installed around the member for fixing the substrate, causing an increase in installation space and cost.